It is known that the so-called submarine phenomenon may occur in an impact situation such as a vehicle crash by the vehicle occupant slipping forward under the seat belt, and the waist belt failing to restrain the pelvis of the vehicle occupant. This is prone to occur when the occupant sits in the front end of the seat or when the back rest is tilted rearward, and reduces the effectiveness of the seat belt in restraining the vehicle occupant or prevents the desired parts of the vehicle occupant from being restrained.
Therefore, it is conceivable to raise the front end of the seat by providing a projection in a front end of the seat frame, or by installing a panel in a front end of the seat frame. However, a desired effect in preventing submarining may not be achieved if the projection or the panel is too low, and the comfort of the vehicle occupant may be impaired if it is too low.
It has been previously proposed to raise the front end of the seat only in case of an impact. Such proposals include those using an air bag (Japanese patent laid-open (kokai) publications No. 5-229378, No. 7-81466, and No. 3-227745), those which mechanically raise the front end of the seat (Japanese UM laid-open (kokai) publications No. 2-149328, No. 3-121947, and No. 4-93222), and those using a pyrotechnic actuator to mechanically raise the front end of the seat (Japanese UM laid-open (kokai) publication No. 3-61440).
A conventional power actuator 50 using high pressure gas is illustrated in FIG. 15. A cylinder 51 fixedly attached to a base member of the seat is provided with an inner bore 51a having a reduced diameter portion 51b in an intermediate part thereof, and receives a piston 52 in one end portion thereof. A free end 52a of the piston 52, which is integrally attached to the main body portion, for instance via a threaded engagement means, projects from an open end of the cylinder 51, and engages a front end of a seat via a link member or the like. The base end 52b of the piston 52 is provided with a somewhat enlarged diameter, and engages the inner circumferential surface of the cylinder inner bore 51a via an O-ring 53. The open one end of the cylinder 51 is fixedly closed by a cap 54 having a central opening 54a. The opening 54a slidably receives an intermediate part of the piston 52 so as to serve as a slide bearing for the movement of the piston 52.
The other end portion of the inner bore 51a of the cylinder 51 receives a propellant 55. The rear open end of the cylinder 51 is crimped on the rear end of the propellant via a retaining plate 57, serving also as a seal, and is thereby closed so as to keep the propellant 55 in place.
The above described structure is typically installed in a vehicle with the piston 52 pushed all the way into the bottom of the inner bore 51a of the cylinder 51. However, if the gap between the base end 52b of the piston 52 and the shoulder of the reduced diameter portion 51b is too small, some difficulty may be encountered in installing the assembly to the vehicle due to possible dimensional errors in the various component parts of the pretensioner device (such as those found in the joints of the link members, the base member of the seat, and the cylinder retainer) and positional errors of the link members. If there is any play in the joint between the free end 52a of the piston 52 and the associated link member, and the joint between the link member and the buckle, impacts resulting from the cumulative effect of such plays may cause local plastic deformations, and energy loss of the high pressure gas.
Also, to prevent the vehicle occupant from being thrown forward in case of a vehicle crash or the like, it has been proposed to fit an automotive seat with a pretensioner device which is adapted to increase the restraint by the seat belt to restrain the forward movement of the vehicle occupant by rapidly pulling a buckle attached to a seat belt in the direction to increase the tension of the seat belt (U.S. Pat. No. 4,705,296).
Japanese patent laid open (kokai) publication No. 10-181527 discloses such a pretensioner device which is provided with a power actuator using a chemically produced high pressure gas for pulling a buckle. Such prior art devices involves some problems due to such plays. It is conceivable to install mechanisms to remove such plays in each device, but such mechanisms tend to increase the size of the device.